Different extracellular domains of the neural cell adhesion molecule (N-CAM) are involved in different functions

Identification of a novel fibroblast growth factor receptor-agonistic peptide and its effect on diabetic wound healing

AIMS

Fibroblast growth factor (FGF) is a broad class of secretory chemicals that act via FGF receptors (FGFR). The study aims to explore the role of a novel peptide, FAP1 (FGFR-agonistic peptide 1), in tissue regeneration and repair. It investigates whether FAP1 mimics basic fibroblast growth factor (bFGF) and accelerates wound healing both in vitro and in vivo.

MAIN METHODS

In this study, a novel peptide was designed and its ability to mimic bFGF was assessed through different in vitro experiments including its effect on cell proliferation, wound healing, cell signaling including FGFR1 phosphorylation and activation of mitogen-activated protein kinases (MAPKs). Specificity was confirmed through surface plasmon resonance (SPR) analysis and co-treatment with FGFR inhibitor, erdafitinib. In vivo, the effect of FAP1 on diabetic wound healing was tested in a mouse model, examining collagen production and the migration and proliferation of keratinocytes and fibroblasts.

KEY FINDINGS

FAP1 specifically phosphorylated FGFR and activated MAPKs similar to bFGF. In vitro, it induced cell proliferation and accelerated wound healing. In vivo, FAP1 improved diabetic wound healing by increasing collagen production and promoting keratinocyte and fibroblast migration and proliferation. The specificity of FAP1 was confirmed through SPR.

SIGNIFICANCE

FAP1 shows potential as a novel pharmacological alternative to natural bFGF for skin tissue regeneration and repair. Its ability to accelerate wound healing and its specificity for FGFR suggest that FAP1 could serve as a cost-effective substitute for bFGF protein in therapeutic applications.

Cell Adhesion Molecules in Schizophrenia Patients with Metabolic Syndrome

Metabolic syndrome (MetS) is a common comorbidity of schizophrenia and significantly shortens life expectancy of the patients. Intercellular (ICAM), vascular (VCAM), and neural (NCAM) cell adhesion molecules (CAMs) mediate neuroinflammatory processes, and their soluble forms (e.g., sICAM) in plasma are present in parallel with their cell-bound forms. In this study, their serum levels were examined in 211 white Siberian patients with paranoid schizophrenia (82 patients with and 129 without MetS according to the 2005 International Diabetes Federation criteria). Serum levels of CAMs were determined with Magpix and Luminex 200 (Luminex, Austin, TX, USA) using xMAP Technology. The level of sICAM-1 was significantly higher and that of sVCAM-1 significantly lower in patients with MetS compared to patients without MetS. Levels of NCAM did not differ between the groups. More pronounced Spearman’s correlations between CAMs, age, duration of schizophrenia, and body–mass index were observed among patients without MetS than among patients with MetS. Our results are consistent with MetS’s being associated with endothelial dysfunction along with other components of inflammation. Through these endothelial components of peripheral inflammatory processes, MetS might induce intracerebral neuroinflammatory changes, but further investigation is needed to confirm this.

Altering Cell-Cell Interaction in Prenatal Alcohol Exposure Models: Insight on Cell-Adhesion Molecules During Brain Development

Alcohol exposure during pregnancy disrupts the development of the brain and produces long lasting behavioral and cognitive impairments collectively known as Fetal Alcohol Spectrum Disorders (FASDs). FASDs are characterized by alterations in learning, working memory, social behavior and executive function. A large body of literature using preclinical prenatal alcohol exposure models reports alcohol-induced changes in architecture and activity in specific brain regions affecting cognition. While multiple putative mechanisms of alcohol’s long-lasting effects on morphology and behavior have been investigated, an area that has received less attention is the effect of alcohol on cell adhesion molecules (CAMs). The embryo/fetal development represents a crucial period for Central Nervous System (CNS) development during which the cell-cell interaction plays an important role. CAMs play a critical role in neuronal migration and differentiation, synaptic organization and function which may be disrupted by alcohol. In this review, we summarize the physiological structure and role of CAMs involved in brain development, review the current literature on prenatal alcohol exposure effects on CAM function in different experimental models and pinpoint areas needed for future study to better understand how CAMs may mediate the morphological, sensory and behavioral outcomes in FASDs.

Competitive coordination of the dual roles of the Hedgehog co-receptor in homophilic adhesion and signal reception

Hedgehog (Hh) signaling patterns embryonic tissues and contributes to homeostasis in adults. In Drosophila, Hh transport and signaling are thought to occur along a specialized class of actin-rich filopodia, termed cytonemes. Here, we report that Interference hedgehog (Ihog) not only forms a Hh receptor complex with Patched to mediate intracellular signaling, but Ihog also engages in trans-homophilic binding leading to cytoneme stabilization in a manner independent of its role as the Hh receptor. Both functions of Ihog (trans-homophilic binding for cytoneme stabilization and Hh binding for ligand sensing) involve a heparin-binding site on the first fibronectin repeat of the extracellular domain. Thus, the Ihog-Ihog interaction and the Hh-Ihog interaction cannot occur simultaneously for a single Ihog molecule. By combining experimental data and mathematical modeling, we determined that Hh-Ihog heterophilic interaction dominates and Hh can disrupt and displace Ihog molecules involved in trans-homophilic binding. Consequently, we proposed that the weaker Ihog-Ihog trans interaction promotes and stabilizes direct membrane contacts along cytonemes and that, as the cytoneme encounters secreted Hh ligands, the ligands trigger release of Ihog from trans Ihog-Ihog complex enabling transport or internalization of the Hh ligand-Ihog-Patched -receptor complex. Thus, the seemingly incompatible functions of Ihog in homophilic adhesion and ligand binding cooperate to assist Hh transport and reception along the cytonemes.

The Drosophila Hedgehog receptor component Interference hedgehog (Ihog) mediates cell-cell interactions through trans-homophilic binding

Hedgehog (Hh) signaling is crucial for establishing complex cellular patterns in embryonic tissues and maintaining homeostasis in adult organs. In Drosophila, Interference hedgehog (Ihog) or its close paralogue Brother of Ihog (Boi) forms a receptor complex with Patched to mediate intracellular Hh signaling. Ihog proteins (Ihog and Boi) also contribute to cell segregation in wing imaginal discs through an unknown mechanism independent of their role in transducing the Hh signal. Here, we report a molecular mechanism by which the Ihog proteins mediate cell-cell interactions. We found that Ihog proteins are enriched at the site of cell-cell contacts and engage in trans-homophilic interactions in a calcium-independent manner. The region that we identified as mediating the trans-Ihog-Ihog interaction overlaps with the Ihog-Hh interface on the first fibronectin repeat of the extracellular domain of Ihog. We further demonstrate that Hh interferes with Ihog-mediated homophilic interactions by competing for Ihog binding. These results, thus, not only reveal a mechanism for Ihog-mediated cell-cell interactions but also suggest a direct Hh-mediated regulation of both intracellular signaling and cell adhesion through Ihog.

The N Terminus of the Prion Protein Mediates Functional Interactions with the Neuronal Cell Adhesion Molecule (NCAM) Fibronectin Domain

The cellular form of the prion protein (PrP^C) is a highly conserved glycoprotein mostly expressed in the central and peripheral nervous systems by different cell types in mammals. A misfolded, pathogenic isoform, denoted as prion, is related to a class of neurodegenerative diseases known as transmissible spongiform encephalopathy. PrP^C function has not been unequivocally clarified, and it is rather defined as a pleiotropic protein likely acting as a dynamic cell surface scaffolding protein for the assembly of different signaling modules. Among the variety of PrP^C protein interactors, the neuronal cell adhesion molecule (NCAM) has been studied in vivo, but the structural basis of this functional interaction is still a matter of debate. Here we focused on the structural determinants responsible for human PrP^C (HuPrP) and NCAM interaction using stimulated emission depletion (STED) nanoscopy, SPR, and NMR spectroscopy approaches. PrP^C co-localizes with NCAM in mouse hippocampal neurons, and this interaction is mainly mediated by the intrinsically disordered PrP^C N-terminal tail, which binds with high affinity to the NCAM fibronectin type-3 domain. NMR structural investigations revealed surface-interacting epitopes governing the interaction between HuPrP N terminus and the second module of the NCAM fibronectin type-3 domain. Our data provided molecular details about the interaction between HuPrP and the NCAM fibronectin domain, and revealed a new role of PrP^C N terminus as a dynamic and functional element responsible for protein-protein interaction.

Exenatide Alters Gene Expression of Neural Cell Adhesion Molecule (NCAM), Intercellular Cell Adhesion Molecule (ICAM), and Vascular Cell Adhesion Molecule (VCAM) in the Hippocampus of Type 2 Diabetic Model Mice

Background

Glucagon-like peptide-1 (GLP-1), a potent and selective agonist for the GLP-1 receptor, ameliorates the symptoms of diabetes through stimulation of insulin secretion. Exenatide is a potent and selective agonist for the GLP-1 receptor. Cell adhesion molecules are members of the immunoglobulin superfamily and are involved in synaptic rearrangements in the mature brain.

Material/Methods

The present study demonstrated the effects of exenatide treatment (0.1 μg/kg, subcutaneously, twice daily for 2 weeks) on the gene expression levels of cell adhesion molecules, neural cell adhesion molecule (NCAM), intercellular cell adhesion molecule (ICAM), and vascular cell adhesion molecule (VCAM) in the brain tissue of diabetic BALB/c male mice by real-time quantitative polymerase chain reaction (PCR). Diabetes was induced by streptozotocin/nicotinamide (STZ-NA) injection to male mice.

Results

The results of this study revealed that hippocampal gene expression of NCAM, ICAM, and VCAM were found to be up-regulated in STZ-NA-induced diabetic mice compared to those of controls. A significant decrease in the gene expression levels of NCAM, ICAM, and VCAM were determined after 2 weeks of exenatide administration.

Conclusions

Cell adhesion molecules may be involved in the molecular mechanism of diabetes. Exenatide has a strong beneficial action in managing diabetes induced by STZ/NA by altering gene expression of NCAM, ICAM, and VCAM.

Cell Adhesion Molecules and Ubiquitination-Functions and Significance

Cell adhesion molecules of the immunoglobulin (Ig) superfamily represent the biggest group of cell adhesion molecules. They have been analyzed since approximately 40 years ago and most of them have been shown to play a role in tumor progression and in the nervous system. All members of the Ig superfamily are intensively posttranslationally modified. However, many aspects of their cellular functions are not yet known. Since a few years ago it is known that some of the Ig superfamily members are modified by ubiquitin. Ubiquitination has classically been described as a proteasomal degradation signal but during the last years it became obvious that it can regulate many other processes including internalization of cell surface molecules and lysosomal sorting. The purpose of this review is to summarize the current knowledge about the ubiquitination of cell adhesion molecules of the Ig superfamily and to discuss its potential physiological roles in tumorigenesis and in the nervous system.

ATP interferes with neural cell adhesion molecule-induced neurite outgrowth

The neural cell adhesion molecule (NCAM) is a member of the immunoglobulin superfamily and involved in the development and regeneration of the nervous system. NCAM is expressed in three major isoforms. Two of them have large intracellular domains of different lengths and are named according to their apparent molecular weight as NCAM140 or NCAM180. NCAM140 is known to promote neurite outgrowth, whereas NCAM180 stabilizes cell-cell contacts. Beside its role in neurite outgrowth, NCAM also shows ATPase activity on its extracellular domain. Here, we quantified neurite outgrowth of neuroblastoma cells, which were transfected with either NCAM140 or NCAM180 in the absence or presence of ATP. We could confirm that NCAM140 strongly promotes neurite outgrowth, whereas NCAM180 stimulates neurite outgrowth only to a minor extent. Furthermore, application of ATP reduced the NCAM-induced neurite outgrowth to background levels in a concentration-dependent and isoform-dependent manner.

Cell adhesion and intracellular calcium signaling in neurons

Cell adhesion molecules (CAMs) play indispensable roles in the developing and mature brain by regulating neuronal migration and differentiation, neurite outgrowth, axonal fasciculation, synapse formation and synaptic plasticity. CAM-mediated changes in neuronal behavior depend on a number of intracellular signaling cascades including changes in various second messengers, among which CAM-dependent changes in intracellular Ca²⁺ levels play a prominent role. Ca²⁺ is an essential secondary intracellular signaling molecule that regulates fundamental cellular functions in various cell types, including neurons. We present a systematic review of the studies reporting changes in intracellular Ca²⁺ levels in response to activation of the immunoglobulin superfamily CAMs, cadherins and integrins in neurons. We also analyze current experimental evidence on the Ca²⁺ sources and channels involved in intracellular Ca²⁺ increases mediated by CAMs of these families, and systematically review the role of the voltage-dependent Ca²⁺ channels (VDCCs) in neurite outgrowth induced by activation of these CAMs. Molecular mechanisms linking CAMs to VDCCs and intracellular Ca²⁺ stores in neurons are discussed.

UCHL1 regulates ubiquitination and recycling of the neural cell adhesion molecule NCAM

The neural cell adhesion molecule (NCAM) is involved in neural development and in plasticity in the adult brain. NCAM140 and NCAM180 isoforms are transmembrane proteins with cytoplasmic domains that differ only in an alternatively spliced exon in the NCAM180 isoform. Both isoforms can interact with several extracellular and cytoplasmic molecules mediating NCAM-dependent functions. Most identified intracellular interaction partners bind to both isoforms, NCAM140 and NCAM180. To identify further intracellular interaction partners specifically binding to NCAM180 the cytosolic domain of human NCAM180 was recombinantly expressed and applied onto a protein macroarray containing the protein library from human fetal brain. We identified the ubiquitin C-terminal hydrolase (UCHL1) which has been described as a de-ubiquitinating enzyme as a potential interaction partner of NCAM180. Since NCAM180 and NCAM140 are ubiquitinated, NCAM140 was included in the subsequent experiments. A partial colocalization of both NCAM isoforms and UCHL1 was observed in primary neurons and the B35 neuroblastoma cell line. Overexpression of UCHL1 significantly decreased constitutive ubiquitination of NCAM180 and NCAM140 whereas inhibition of endogenous UCHL1 increased NCAM's ubiquitination. Furthermore, lysosomal localization of NCAM180 and NCAM140 was significantly reduced after overexpression of UCHL1 consistent with a partial colocalization of internalized NCAM with UCHL1. These data indicate that UCHL1 is a novel interaction partner of both NCAM isoforms that regulates their ubiquitination and intracellular trafficking.

Neuritogenic and neuroprotective properties of peptide agonists of the fibroblast growth factor receptor

Fibroblast growth factor receptors (FGFRs) interact with their cognate ligands, FGFs, and with a number of cell adhesion molecules (CAMs), such as the neural cell adhesion molecule (NCAM), mediating a wide range of events during the development and maintenance of the nervous system. Determination of protein structure, in silico modeling and biological studies have recently resulted in the identification of FGFR binding peptides derived from various FGFs and NCAM mimicking the effects of these molecules with regard to their neuritogenic and neuroprotective properties. This review focuses on recently developed functional peptide agonists of FGFR with possible therapeutic potential.

Specific detection of CD56 (NCAM) isoforms for the identification of aggressive malignant neoplasms with progressive development

Alternative splicing of transcripts from many cancer-associated genes is believed to play a major role in carcinogenesis as well as in tumor progression. Alternative splicing of one such gene, the neural cell adhesion molecule CD56 (NCAM), impacts the progression, inadequate therapeutic response, and reduced total survival of patients who suffer from numerous malignant neoplasms. Although previous investigations have determined that CD56 exists in three major isoforms (CD56(120kD), CD56(140kD), and CD56(180kD)) with individual structural and functional properties, neither the expression profiles nor the functional relevance of these isoforms in malignant tumors have been consistently investigated. Using new quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) strategies and novel CD56 isoform-specific antibodies, CD56(140kD) was shown to be exclusively expressed in a number of highly malignant CD56(+) neoplasms and was associated with the progression of CD56(+) precursor lesions of unclear malignant potential. Moreover, only CD56(140kD) induced antiapoptotic/proliferative pathways and specifically phosphorylated calcium-dependent kinases that are relevant for tumorigenesis. We conclude, therefore, that the specific detection of CD56 isoforms will help to elucidate their individual functions in the pathogenesis and progression of malignant neoplasms and may have a positive impact on the development of CD56-based immunotherapeutic strategies.

Fibroblast growth factor-derived peptides: functional agonists of the fibroblast growth factor receptor

A series of peptides, termed dekafins, were derived from the beta10-beta11 loop regions of fibroblast growth factors (FGFs) 1, 2, 3, 5, 6, 8, 9, 10, and 17. The dekafins share a homologous amino acid sequence similar to a sequence in the first fibronectin type III module of the neural cell adhesion molecule. All dekafins were shown by surface plasmon resonance analysis to bind fibroblast growth factor receptor (FGFR)1-IIIc-Ig2-3 and FGFR2-IIIb-Ig2-3, respectively, with K(d) values of approximately 10(-7) to 10(-8) mol/L. Binding of dekafin1 to FGFR1-IIIc-Ig2-3 was inhibited by a heparin analog, sucrose octasulfate, indicating that heparin sulfate moiety can modulate dekafin binding to FGFRs. Treatment of transcription and mRNA export (TREX) cells permanently expressing Strep-tag-labeled FGFR1-IIIc with dekafins resulted in receptor phosphorylation. FGF1-induced FGFR1-IIIc phosphorylation was inhibited by dekafin1 and 10 in high concentrations, indicating that dekafins are FGFR partial agonists. The dekafins induced neuronal differentiation as reflected by neurite outgrowth from cerebellar granule neurons, an effect that was abolished by SU5402, a specific inhibitor of the FGFR tyrosine kinase, and by inositolhexaphosphate, an extracellularly acting FGFR antagonist. Some, but not all, dekafins were capable of promoting survival of cerebellar granule neurons induced to undergo apoptosis. Thus, the dekafins are functional FGFR agonists with apparent therapeutic potential.

Pharmacology of cell adhesion molecules of the nervous system

Cell adhesion molecules (CAMs) play a pivotal role in the development and maintenance of the nervous system under normal conditions. They also are involved in numerous pathological processes such as inflammation, degenerative disorders, and cancer, making them attractive targets for drug development. The majority of CAMs are signal transducing receptors. CAM-induced intracellular signalling is triggered via homophilic (CAM-CAM) and heterophilic (CAM - other counter-receptors) interactions, which both can be targeted pharmacologically. We here describe the progress in the CAM pharmacology focusing on cadherins and CAMs of the immunoglobulin (Ig) superfamily, such as NCAM and L1. Structural basis of CAM-mediated cell adhesion and CAM-induced signalling are outlined. Different pharmacological approaches to study functions of CAMs are presented including the use of specific antibodies, recombinant proteins, and synthetic peptides. We also discuss how unravelling of the 3D structure of CAMs provides novel pharmacological tools for dissection of CAM-induced signalling pathways and offers therapeutic opportunities for a range of neurological disorders.

Contactin1a expression is associated with oligodendrocyte differentiation and axonal regeneration in the central nervous system of zebrafish

Contactin1a (Cntn1a) is a zebrafish homolog of contactin1 (F3/F11/contactin) in mammals, an immunoglobulin superfamily recognition molecule of neurons and oligodendrocytes. We describe conspicuous Cntn1a mRNA expression in oligodendrocytes in the developing optic pathway of zebrafish. In adults, this expression is only retained in glial cells in the intraretinal optic fiber layer, which contains 'loose' myelin. After optic nerve lesion, oligodendrocytes re-express Cntn1a mRNA independently of the presence of regenerating axons and retinal ganglion cells upregulate Cntn1a expression to levels that are significantly higher than those during development. After spinal cord lesion, expression of Cntn1a mRNA is similarly increased in axotomized brainstem neurons and white matter glial cells in the spinal cord. In addition, reduced mRNA expression in the trigeminal/anterior lateral line ganglion in erbb3-deficient mutant larvae implies Cntn1a in Schwann cell differentiation. These complex regulation patterns suggest roles for Cntn1a in myelinating cells and neurons particularly in successful CNS regeneration.

Differential activities in adhesion and neurite growth of fibronectin type III repeats in the PTP-delta extracellular domain

The full-length extracellular domain (ECD) of protein tyrosine phosphatase delta (PTP-delta) functions as a ligand to promote cell adhesion and neurite outgrowth; this ECD contains three immunoglobulin (Ig) repeats and eight fibronectin type III (FN III) repeats. However, it is not known which regions of the ECD regulate its ligand functions. Therefore, we constructed and expressed a fusion protein of the PTP-delta ECD lacking FN III repeats 4-8, and tested this protein for neuronal adhesion and neurite-promoting ability. Compared to the full-length isoform, the truncated ECD was poorer at promoting adhesion, but a more potent promoter of neurite growth. The results suggest that distal FN III repeats of PTP-delta are important in adhesive functions, but dispensable for neurite outgrowth promotion. As the predominant isoform of PTP-delta during neural development (type D) also lacks distal FN III repeats, the functional properties we observe may be relevant to periods of axon extension, suggesting that splice variants of receptor PTPs play distinct roles in neural development.

A peptide from the first fibronectin domain of NCAM acts as an inverse agonist and stimulates FGF receptor activation, neurite outgrowth and survival

Neural cell adhesion molecule (NCAM) contributes to axon growth and guidance during development and learning and memory in adulthood. Although the Ig domains mediate homophilic binding, outgrowth activity localizes to two membrane proximal fibronectin-like domains. The first of these contains a site identified as a potential FGF receptor (FGFR) activation motif (FRM) important for NCAM stimulation of neurite outgrowth, but its activity has hitherto remained hypothetical. Here, we have tested the effects of a domain-specific antibody and peptides corresponding to the FRM in cellular assays in vitro. The first fibronectin domain antibody inhibited NCAM-stimulated outgrowth, indicating the importance of the domain for NCAM function. Monomeric FRM peptide behaved as an inverse agonist; low concentrations specifically inhibited neurite outgrowth stimulated by NCAM and cellular responses to FGF2, while saturating concentrations stimulated FGFR-dependent neurite outgrowth equivalent to NCAM itself. Dendrimeric FRM peptide was 125-fold more active and stimulated FGFR activation, FGFR-dependent and FGF-mimetic neurite outgrowth and cell survival (but not proliferation). We conclude that the FRM peptide contains NCAM-mimetic bioactivity accounted for by stimulation of FGF signalling pathways at the level of or upstream from FGF receptors, and discuss the possibility that FRM comprises part of an FGFR activation site on NCAM.

Altered expression of NCAM in hippocampus and cortex may underlie memory and learning deficits in rats with streptozotocin-induced diabetes mellitus

Neurological and structural changes are paralleled by cognitive deficits in diabetes mellitus. The present study was designed to evaluate the expression of neural cell adhesion molecules (NCAM) in the hippocampus, cortex and cerebellum and to examine cognitive functions in diabetic rats. Diabetes was induced in male albino rats via intraperitoneal streptozotocin injection. Learning and memory behaviors were investigated using a passive avoidance test and a spatial version of the Morris water maze test. NCAM expression was detected in the hippocampus, cortex and cerebellum by an immunoblotting method. The diabetic rats developed significant impairment in learning and memory behaviours as indicated by deficits in passive avoidance and water maze tests as compared to control rats. Expression of NCAM 180 and 120 kDa were found to be higher in hippocampus and cortex of diabetic rat brains compared to those of control, whereas expression of NCAM 140 kDa decreased in these brain regions. Our findings suggest that streptozotocin-induced diabetes impairs cognitive functions and causes an imbalance in expression of NCAM in those brain regions involved in learning and memory. Altered expression of NCAM in hippocampus may be an important cause of learning and memory deficits that occur in diabetes mellitus.

Structural basis for a direct interaction between FGFR1 and NCAM and evidence for a regulatory role of ATP

The neural cell adhesion molecule (NCAM) promotes axonal outgrowth, presumably through an interaction with the fibroblast growth factor receptor (FGFR). NCAM also has a little-understood ATPase activity. We here demonstrate for the first time a direct interaction between NCAM (fibronectin type III [F3] modules 1 and 2) and FGFR1 (Ig modules 2 and 3) by surface plasmon resonance (SPR) analysis. The structure of the NCAM F3 module 2 was determined by NMR and the module was shown by NMR to interact with the FGFR1 Ig module 3 and ATP. The NCAM sites binding to FGFR and ATP were found to overlap and ATP was shown by SPR to inhibit the NCAM-FGFR binding, indicating that ATP probably regulates the NCAM-FGFR interaction. Furthermore, we demonstrate that the NCAM module was able to induce activation (phosphorylation) of FGFR and to stimulate neurite outgrowth. In contrast, ATP inhibited neurite outgrowth induced by the module.

The immunoglobulin-superfamily molecule basigin is a binding protein for oligomannosidic carbohydrates: an anti-idiotypic approach

Recognition molecules that carry carbohydrate structures regulate cell interactions during development and play important roles in synaptic plasticity and regeneration in the adult. Glycans appear to be involved in these interactions. We have searched for binding proteins for oligomannosidic structures using the L3 antibody directed against high mannose-type glycans in an anti-idiotypic approach. A selected monoclonal anti-idiotype antibody was used for affinity chromatography and identified basigin as a binding protein from mouse brain detergent lysates. Basigin was found to bind to high mannose-carrying cell recognition molecules, such as myelin-associated glycoprotein, L1, the beta2-subunit of Na+/K+-ATPase and an oligomannosidic neoglycolipid. Furthermore, basigin was involved in outgrowth of astrocytic processes in vitro. A striking homology between the first immunoglobulin (Ig)-like domain of basigin and the fourth Ig-like domain of NCAM, previously shown to bind to oligomannosidic glycans, and the lectin domain of the mannose receptor confirms that basigin is an oligomannose binding lectin. To our knowledge this is the first report that anti-idiotypic antibodies can be used to identify binding partners for carbohydrates.

Expression of protein zero is increased in lesioned axon pathways in the central nervous system of adult zebrafish

The immunoglobulin superfamily molecule protein zero (P0) is important for myelin formation and may also play a role in adult axon regeneration, since it promotes neurite outgrowth in vitro. Moreover, it is expressed in the regenerating central nervous system (CNS) of fish, but not in the nonregenerating CNS of mammals. We identified a P0 homolog in zebrafish. Cell type-specific expression of P0 begins in the ventromedial hindbrain and the optic chiasm at 3-5 days of development. Later (at 4 weeks) expression has spread throughout the optic system and spinal cord. This is consistent with a role for P0 in CNS myelination during development. In the adult CNS, glial cells constitutively express P0 mRNA. After an optic nerve crush, expression is increased within 2 days in the entire optic pathway. Expression peaks at 1 to 2 months and remains elevated for at least 6 months postlesion. After enucleation, P0 mRNA expression is also upregulated but fails to reach the high levels observed in crush-lesioned animals at 4 weeks postlesion. Spinal cord transection leads to increased expression of P0 mRNA in the spinal cord caudal to the lesion site. The glial upregulation of P0 mRNA expression after a lesion of the adult zebrafish CNS suggests roles for P0 in promoting axon regeneration and remyelination after injury.

Characterization of a novel NCAM ligand with a stimulatory effect on neurite outgrowth identified by screening a combinatorial peptide library

The neural cell adhesion molecule, NCAM, plays a key role in neural development and plasticity mediating cell adhesion and signal transduction. By screening a combinatorial library of synthetic peptides with NCAM purified from postnatal day 10 rat brains, we identified a nonapeptide, termed NCAM binding peptide 10 (NBP10) and showed by nuclear magnetic resonance analysis that it bound the NCAM IgI module of NCAM. NBP10 modulated cell aggregation as well as neurite outgrowth induced specifically by homophilic NCAM binding. Moreover, both monomeric and multimeric forms of NBP10 stimulated neurite outgrowth from primary hippocampal neurons. The neurite outgrowth response to NBP10 was inhibited by a number of compounds previously shown to inhibit neurite outgrowth induced by homophilic NCAM binding, including voltage-dependent calcium channel antagonists, suggesting that NBP10 induced neurite outgrowth by activating a signal transduction pathway similar to that activated by NCAM itself. Moreover, an inhibitor of intracellular calcium mobilization, TMB-8, prevented NBP10-induced neurite outgrowth suggesting that NCAM-dependent neurite outgrowth also requires mobilization of calcium from intracellular calcium stores in addition to calcium influx from extracellular sources. By single-cell calcium imaging we further demonstrated that NBP10 was capable of inducing an increase in intracellular calcium in PC12E2 cells. Thus, the NBP10 peptide is a new tool for the study of molecular mechanisms underlying NCAM-dependent signal transduction and neurite outgrowth, and could prove to be a useful modulator of regenerative processes in the peripheral and central nervous system.

Cell Type-specific Effects of the Neural Adhesion Molecules L1 and N-CAM on Diverse Second Messenger Systems

We have previously shown that the neural adhesion molecules L1 and N-CAM influence second messenger systems when triggered with specific antibodies at the surface of the phaeochromocytoma PC12 cell line (Schuch et al., Neuron, 3, 13 - 20, 1989). To determine whether the two molecules are linked to the same intracellular signalling cascades, independent of the cell type expressing them, or whether different neural cell types respond with different signal transduction mechanisms, we have investigated the effects of antibodies to L1 and N-CAM, and the isolated molecules themselves, on second messenger systems in different neural cell types. We have investigated cultures of cerebellar and dorsal root ganglion neurons and transformed Schwann cells and related these results to those obtained with the PC12 cell line. Here we show that addition of L1 and N-CAM antibodies and the isolated molecules themselves elicit cell type-specific responses that can be modulated by the substrate on which the cells are maintained. Depending on the cell type, cells respond to the triggering of L1 and N-CAM with antibodies, or addition of the purified molecules, by either up-regulation or down-regulation of inositol phosphate turnover, by a rise in intracellular Ca2+ levels dependent on or independent of the opening of voltage-gated Ca2+ channels, or by an increase or decrease in intracellular pH. Moreover, cerebellar neurons expressing N-CAM respond to addition N-CAM, but not to N-CAM antibodies, in contrast to the other neural cell types studied, which respond to both triggers. Furthermore, cerebellar neurons were the only cells to show a rise in cAMP levels in response to any of the ligands tested. This stimulation of cAMP production by L1 antibodies depended on the cross-linking of L1 molecules at the cell surface, whereas the other responses did not depend on clustering of L1. Simultaneous addition of L1 and N-CAM antibodies either elicited an additive or more than additive effect on the intracellular responses which, for cerebellar neurons, depends on the substrate on which the cells are maintained. These observations indicate that L1 and N-CAM or their antibodies activate cell type-specific intracellular signalling systems and that the two molecules can act interdependently or independently of each other.

Direct measurement of local raised subplasmalemmal calcium concentrations in growth cones advancing on an N-cadherin substrate

We have used the membrane-localized calcium probe fura-piperazine-C12H25 (FFP-18) to examine cytosolic calcium concentrations in a volume close to the plasmalemma. Although promotion of axon outgrowth by cell adhesion molecules requires extracellular calcium and is correlated with an opening of plasmalemmal channels, conventional indicators cannot detect a change in the calcium concentration in such stimulated growth cones. We have examined calcium signalling in chick retinal ganglion cell growth cones extending along stripes of N-cadherin. Subplasmalemmal calcium concentrations, reported by FFP-18, were significantly higher in these growth cones than in neighbouring growth cones on either fibronectin or polylysine. In contrast, the bulk cytosolic calcium concentration throughout the growth cone, as measured by Fura-2, was identical in growth cones on and off the N-cadherin stripes. Our results suggest that guidance cues can use extremely local calcium signals to control pathfinding decisions.

Increased intracellular calcium is required for neurite outgrowth induced by a synthetic peptide ligand of NCAM

We have recently identified a synthetic peptide, termed C3, capable of binding the first immunoglobulin-like module of neural cell adhesion molecule (NCAM) by means of combinatorial chemistry and shown that this NCAM ligand promotes neurite outgrowth. By means of single cell calcium imaging using the calcium-sensitive probe fura-2-acetomethyl ester, we here show that the C3-peptide induced an increase in intracellular calcium in primary hippocampal neurons and PC12-E2 cells, presumably requiring mobilization of calcium from both extracellular and intracellular stores. We further observed that C3-induced neurite outgrowth was inhibited by antagonists of voltage-dependent calcium channels as well as by an inhibitor of intracellular calcium mobilization, TMB-8. These findings demonstrate at the single cell level that a synthetic NCAM ligand directly can induce an increase in intracellular calcium and suggest that NCAM-dependent neurite outgrowth requires calcium mobilization from both extracellular and intracellular calcium stores. Thus, the C3-peptide may be regarded as a useful tool for the study of NCAM-dependent signal transduction. Furthermore, the peptide may be of considerable therapeutical interest for the treatment of neurodegenerative disorders.

CRYP-2/cPTPRO is a neurite inhibitory repulsive guidance cue for retinal neurons in vitro

Receptor protein tyrosine phosphatases (RPTPs) are implicated as regulators of axon growth and guidance. Genetic deletions in the fly have shown that type III RPTPs are important in axon pathfinding, but nothing is known about their function on a cellular level. Previous experiments in our lab have identified a type III RPTP, CRYP-2/cPTPRO, specifically expressed during the period of axon outgrowth in the chick brain; cPTPRO is expressed in the axons and growth cones of retinal and tectal projection neurons. We constructed a fusion protein containing the extracellular domain of cPTPRO fused to the Fc portion of mouse immunoglobulin G-1, and used it to perform in vitro functional assays. We found that the extracellular domain of cPTPRO is an antiadhesive, neurite inhibitory molecule for retinal neurons. In addition, cPTPRO had potent growth cone collapsing activity in vitro, and locally applied gradients of cPTPRO repelled growing retinal axons. This chemorepulsive effect could be regulated by the level of cGMP in the growth cone. Immunohistochemical examination of the retina indicated that cPTPRO has at least one heterophilic binding partner in the retina. Taken together, our results indicate that cPTPRO may act as a guidance cue for retinal ganglion cells during vertebrate development.

Molecular defects that cause loss of polysialic acid in the complementation group 2A10

Polysialic acid (PSA) is a dynamically regulated posttranslational modification of the neural cell adhesion molecule (NCAM), which modulates NCAM binding functions. PSA biosynthesis is catalyzed by two polysialyltransferases, ST8SiaII and ST8SiaIV. The catalytic mechanisms of these enzymes are unknown. In Chinese hamster ovary cells, ST8SiaIV is responsible for PSA expression. In the complementation group 2A10, the ST8SiaIV gene is disrupted. Investigating the molecular defects in this complementation group, seven clones with missense mutations in ST8SiaIV were found. Mutations cause replacement of amino acids that are highly conserved in alpha2,8-sialyltransferases. To verify the physiological relevance of identified mutations, identical amino acid substitutions were introduced into epitope-tagged variants of hamster ST8SiaIV and murine ST8SiaII and recombinant proteins were tested in vivo and in vitro. None of these constructs reconstituted PSA synthesis in 2A10 cells, although the proteins were expressed and with the exception of the cysteine variants ST8SiaIV-C356F and ST8SiaII-C371F correctly targeted to the Golgi apparatus. Interestingly, two mutations (ST8SiaIV-R277G and -M333V and the corresponding mutants ST8SiaII-R292G and -M348V) could be partially rescued if tested in vitro. Although these mutants were negative for autopolysialylation, partial reconstitution of both auto- and NCAM polysialylation was achieved in the presence of NCAM. The data presented in this study suggest a functional link between auto- and NCAM polysialylation.

Selective expression of neural cell adhesion molecule (NCAM)-180 in optic nerve head astrocytes exposed to elevated hydrostatic pressure in vitro

Glaucomatous optic neuropathy is usually associated with elevated intraocular pressure. Optic nerve head astrocytes may respond to intraocular pressure by stimulation of pressure-sensitive mechanoreceptors on the cell surface. Neural cell adhesion molecule (NCAM) a transmembrane protein, mediates cell adhesion and migration. The NCAM 180 isoform increases in astrocytes of glaucomatous optic nerve head. We characterized the relative expression of NCAM isoforms in human optic nerve head astrocytes grown under elevated hydrostatic pressure. Astrocytes cultured from normal human optic nerve heads were exposed to either atmospheric or continuous hydrostatic pressure of 60 mm Hg, and analyzed at 6-48 h. Changes in cell shape, immunoreactivity, and distribution of GFAP, actin and NCAM were observed in pressure-treated cultures. Newly synthesized (35)S-labeled NCAM protein immunoprecipitated from cell lysates was increased 2-fold within 24 h after exposure to elevated pressure compared to control. The increase in NCAM synthesis was primarily due to the NCAM 180 isoform. A significant increase in NCAM 180 mRNA levels was detected by RT-PCR and Northern blots in cultured optic nerve head astrocytes within 6 h after exposure to elevated pressure. NCAM 180 mRNA and protein synthesis decreased after 24 h and returned to control levels by 48 h. Our data indicate that NCAM 180 transcription and synthesis in astrocytes is stimulated by elevated hydrostatic pressure. Because NCAM 180 interacts with the cytoskeleton through an extended cytoplasmic tail, a selective and transient increase in NCAM 180 in optic nerve head astrocytes exposed to elevated pressure may be relevant to the migration and interactions of reactive astrocytes in glaucoma.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

The role of basement membrane proteins on the expression of neural cell adhesion molecule (N-CAM) in an adenoid cystic carcinoma cell line

We investigated the presence of neural cell adhesion molecule (N-CAM) in the adenoid cystic carcinoma cell line CAC2 using immunofluorescence microscopy. Additionally, we analysed the role of laminin and type IV collagen in N-CAM expression. We demonstrated that cultured adenoid cystic carcinoma cells express N-CAM. Control cells presented a scattered N-CAM expression on cell membrane, and type IV collagen had no effect in N-CAM distribution. CAC2 cells grown on laminin-coated coverslips expressed N-CAM concentrated on cell lamellipodia suggesting relationship with migratory activity. Our results showed that cultured adenoid cystic carcinoma cells express N-CAM and this expression is modulated by extracellular matrix.

Extracellular adenosine triphosphate affects neural cell adhesion molecule (NCAM)-mediated cell adhesion and neurite outgrowth

The neural cell adhesion molecule (NCAM) plays an important role in synaptic plasticity in embryonic and adult brain. Recently, it has been demonstrated that NCAM is capable of binding and hydrolyzing extracellular ATP. The purpose of the present study was to evaluate the role of extracellular ATP in NCAM-mediated cellular adhesion and neurite outgrowth. We here show that extracellularly added adenosine triphosphate (ATP) and its structural analogues, adenosine-5'-O-(3-thiothiophosphate), beta, gamma-methylenadenosine-5'-triphosphate, beta, gamma-imidoadenosine-5-triphosphate, and UTP, in varying degrees inhibited aggregation of hippocampal neurons. Rat glial BT4Cn cells are unable to aggregate when grown on agar but acquire this capacity when transfected with NCAM. However, addition of extracellular ATP to NCAM-transfected BT4Cn cells inhibited aggregation. Furthermore, neurite outgrowth from hippocampal neurons in cultures allowing NCAM-homophilic interactions was inhibited by addition of extracellular nucleotides. These findings indicate that NCAM-mediated adhesion may be modulated by extracellular ATP. Moreover, extracellularly added ATP stimulated neurite outgrowth from hippocampal neurons under conditions non-permissive for NCAM-homophilic interactions, and neurite outgrowth stimulated by extracellular ATP could be inhibited by a synthetic peptide corresponding to the so-called cell adhesion molecule homology domain (CHD) of the fibroblast growth factor receptor (FGFR) and by FGFR antibodies binding to this domain. Antibodies against the fibronectin type-III homology modules of NCAM, in which a putative site for ATP binding and hydrolysis is located, also abolished the neurite outgrowth-promoting effect of ATP. The non-hydrolyzable analogues of ATP all strongly inhibited neurite outgrowth. Our results indicate that extracellular ATP may be involved in synaptic plasticity through a modulation of NCAM-mediated adhesion and neurite outgrowth.

The neural cell adhesion molecule is a receptor for rabies virus

Previous reports strongly suggest that, in addition to the nicotinic acetylcholine receptor, rabies virus can use other, as-yet-unidentified receptors. We found that laboratory cell lines susceptible to rabies virus infection express the neural cell adhesion molecule (NCAM) (CD56) on their surface, whereas resistant cells do not, supporting the idea that NCAM could be a rabies virus receptor. We observed that (i) incubation with rabies virus decreases the surface expression of NCAM; (ii) treatment of susceptible cells with heparan sulfate, a ligand for NCAM, or with NCAM antibodies significantly reduces the rabies virus infection; and (iii) preincubation of rabies virus inoculum with soluble NCAM protein as a receptor decoy drastically neutralizes the capacity of rabies virus to infect susceptible cells. Moreover, we demonstrated that transfection of resistant L fibroblasts with the NCAM-encoding gene induces rabies virus susceptibility whereas absence of NCAM in the primary cortical cell cultures prepared from NCAM-deficient mice reduces the rabies virus infection and virus production. This provides evidence that NCAM is an in vitro receptor for the rabies virus. Moreover, the in vivo relevance for the use of NCAM as a receptor was demonstrated by the infection of NCAM-deficient mice, in which rabies mortality was delayed and brain invasion by rabies virus was drastically restricted. Our results showed that NCAM, which is expressed mainly in the adult nervous system, plays an important role in rabies infection. However, it cannot be excluded that receptors other than NCAM are utilized. Thus, the description of NCAM as a new rabies virus receptor would be another example of the use by viruses of more than one receptor to gain entry into the host.

Identification and characterization of RPTP rho, a novel RPTP mu/kappa-like receptor protein tyrosine phosphatase whose expression is restricted to the central nervous system

We describe the cloning, chromosomal localization and characterization of RPTPrho, a new member of the RPTPmu/kappa phosphatase subfamily. Receptor tyrosine phosphatases in this subfamily are comprised of a MAM domain near the N-terminal, an immunoglobulin-like domain, four fibronectin type III repeats, a single transmembrane domain, and a large juxtamembrane segment followed by two intracellular phosphatase domains. An alternatively spliced mini-exon was identified in the extracellular segment of RPTPrho, between the fourth fibronectin type III repeat and the transmembrane domain. The RPTPrho gene was mapped to human chromosome 20 and mouse chromosome 2. Northern blot analysis demonstrated that RPTPrho expression was restricted to the central nervous system, and in situ hybridization studies showed that the RPTPrho transcript was distributed throughout the murine brain and spinal cord. Exceptionally high levels of the transcript were present in the cortex and olfactory bulbs during perinatal development, but were down-regulated during postnatal week two. The motifs found in the extracellular segment of type II receptor protein tyrosine phosphatases are commonly found in neural cell adhesion molecules, suggesting that RPTPrho may be involved in both signal transduction and cellular adhesion in the central nervous system.

The role of glycoproteins in neural development function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.

Limbic system-associated membrane protein (LAMP) induces neurite outgrowth and intracellular Ca2+ increase in primary fetal neurons

The ability of cell adhesion molecules (CAMs) to transduce cell surface signals into intracellular responses is critical for developing neurons, particularly during axonal pathfinding and targeting. It has been suggested that different CAMs can promote neuronal outgrowth via activation of common neuronal CAM-specific second-messenger pathways, although the elements involved in this cascade could differ. Limbic system-associated membrane protein (LAMP), a member of the Ig superfamily, is a molecule that promotes cell adhesion and neurite outgrowth from specific populations of fetal neurons. In the present study, we show that LAMP can induce several types of calcium (Ca2+) signals. Neurite outgrowth is promoted if fetal hippocampal neurons are grown on lamp-transfected CHO cells. This LAMP-induced outgrowth of neurons is mediated in part through activation of L-type Ca channels. Application of soluble LAMP to cultures of fetal hippocampal neurons caused a sustained (up to 60 min) elevation of intracellular Ca2+ as measured by fluo-3 fluorescence on a confocal microscope. The number of responding hippocampal neurons was initially low, but increased with age in culture and the [Ca2+]i elevation was only partially decreased by an L-type Ca(2+)-channel blocker. In contrast, at all times in culture, only a small fraction of neurons from visual cortex responded to LAMP application and only with transient elevation of cytosolic Ca2+ (< 15 min). Based on these observations, LAMP appears to function primarily through homophilic interactions and acts in part by modulating intracellular Ca2+ levels during neurite outgrowth by increasing the Ca2+ influx through L-type calcium channels, but has additional effects on intracellular Ca2+ signaling at later developmental stages.

VASE-encoded peptide modifies NCAM- and L1-mediated neurite outgrowth

Interactions between the neural cell adhesion molecule (NCAM) with NCAM-expressing neurons (trans-interaction) stimulate outgrowth of neurites. The extent of NCAM-triggered neurite outgrowth depends on the presence of 10 amino acids derived from the variable alternatively spliced exon (VASE or pi-exon) in the fourth immunoglobulin-like domain of NCAM (Ig4): NCAM with VASE reduces and without VASE enhances neurite outgrowth in cis- or trans-interaction. We have investigated the role of VASE in neurite outgrowth by characterizing the receptors at the cell surface of cultured cerebellar neurons. Results from experiments with L1 and NCAM antibodies and with cerebellar neurons derived from wild-type or NCAM-deficient mice show that substrate-coated Ig4 with VASE (Ig4+) or without VASE (Ig4-) stimulates neurite outgrowth by a trans-interaction with L1 and that Ig4- promotes neurite outgrowth more strongly than Ig4+ by a transinteraction with NCAM.

The first immunoglobulin-like neural cell adhesion molecule (NCAM) domain is involved in double-reciprocal interaction with the second immunoglobulin-like NCAM domain and in heparin binding

To study the function of the first immunoglobulin (Ig)-like domain of the neural cell adhesion molecule (NCAM), it was produced as a recombinant fusion protein in a bacterial expression system and as a recombinant protein in a eukaryotic expression system of the yeast Pichia pastoris. For comparison, other NCAM domains were also produced as fusion proteins. By means of surface plasmon resonance analysis, it was shown that the first Ig-like NCAM domain binds the second Ig-like NCAM domain with a dissociation constant 5.5 +/- 1.6 x 10(-5) M. Furthermore, it was found that the first Ig-like domain binds heparin. It was also demonstrated that the second Ig-like NCAM domain binds heparin and that both domains bind collagen type I via heparin but not collagen type I directly.

The fibronectin domains of the neural adhesion molecule TAX-1 are necessary and sufficient for homophilic binding

Cell adhesion molecules belonging to the immunoglobulin superfamily promote cell aggregation and neurite outgrowth. These proteins are multidomain molecules comprising a number of distinct modules, notably Ig domains of the C2 class and fibronectin type III repeats. A subgroup of these neural adhesion molecules are linked to the membrane with a glycosylphosphatidylinositol anchor and show a more restricted pattern of expression in the embryo. Among them, the human homologue of the transient axonal glycoprotein, named TAX-1, shares a great degree of similarity at the protein level with rodent TAG-1. In the present study we set out to determine which domains of TAX-1 are involved in promoting the homophilic, adhesive properties of the molecule. We established stable Schneider-2 cell lines expressing the intact molecule, the fibronectin, or the immunoglobulin domains. The fibronectin domains were necessary and sufficient to mediate homophilic binding and induce cell aggregation, a response also observed with cells expressing the intact TAX-1 molecule. Aggregation was inhibited by the secreted form of the TAG-1 protein. On the other hand, the immunoglobulin domains by themselves were not able to induce cell aggregation. In addition, TAX-1 was localized in areas of cell contact among aggregating cells, justifying its role as an adhesion molecule.

Cell-type-specific expression of neural cell adhesion molecule (N-CAM) in Ito cells of rat liver. Up-regulation during in vitro activation and in hepatic tissue repair

Ito cells (lipocytes, stellate cells) are regarded as the principle matrix-producing cell of the liver and have been shown recently to express glial fibrillary acidic protein, an intermediate filament typically found in glia cells of the nervous system. The present study examines 1) whether Ito cells of rat liver express central nervous system typical adhesion molecules, namely, neural cell adhesion molecule (N-CAM), in a cell-type-specific manner and 2) whether N-CAM expression is affected by activation of Ito cells in vitro and during rat liver injury in vivo. As assessed by reverse transcriptase polymerase chain reaction, Northern blotting, Western blotting, and immunocytochemistry of freshly isolated and cultivated hepatic cells, N-CAM expression was restricted to Ito cells and was absent in hepatocytes, Kupffer cells, and sinusoidal endothelial cells. Ito cells expressed predominantly N-CAM-coding transcripts of 6.1 and 4.8 kb in size and 140-kd isoforms of the N-CAM protein, which was localized on the cell surface membrane of Ito cells. In parallel to glial fibrillary acidic protein down-regulation and smooth muscle alpha-actin up-regulation, N-CAM expression was increased during in vitro transformation of Ito cells from resting to activated (myofibroblast-like) cells and by the fibrogenic mediator transforming growth factor-beta 1. By immunohistochemistry, N-CAM was detected in normal rat liver in the portal field as densely packed material and in a spot as well as fiber-like pattern probably representing nerve structures. However, after liver injury, N-CAM expression became detectable in mesenchymal cells within and around the necrotic area and within fibrotic septae. In serially cut tissue sections, N-CAM-positive cells were predominantly co-distributed with smooth muscle alpha-actin-positive cells rather than glial fibrillary acidic protein-positive cells, especially in fibrotic livers. The experimental results illustrate that N-CAM positivity in the liver cannot be solely ascribed to nerve endings as, among the different types of resident liver cells, Ito cells specifically express N-CAM in vitro and presumably in vivo. In addition to its role as potential cell-type-specific marker protein for activated Ito cells, the induction of N-CAM expression might illustrate a mechanism by which mesenchymal cell proliferation might be inhibited when tissue repair is concluded.

Structural features of a close homologue of L1 (CHL1) in the mouse: a new member of the L1 family of neural recognition molecules

We have identified a close homologue of L1 (CHL1) in the mouse. CHL1 comprises an N-terminal signal sequence, six immunoglobulin (Ig)-like domains, 4.5 fibronectin type III (FN)-like repeats, a transmembrane domain and a C-terminal, most likely intracellular domain of approximately 100 amino acids. CHL1 is most similar in its extracellular domain to chicken Ng-CAM (approximately 40% amino acid identity), followed by mouse L1, chicken neurofascin, chicken Nr-CAM, Drosophila neuroglian and zebrafish L1.1 (37-28% amino acid identity), and mouse F3, rat TAG-1 and rat BIG-1 (approximately 27% amino acid identity). The similarity with other members of the Ig superfamily [e.g. neural cell adhesion molecule (N-CAM), DCC, HLAR, rse] is 16-11%. The intracellular domain is most similar to mouse and chicken Nr-CAM, mouse and rat neurofascin (approximately 60% amino acid identity) followed by chicken neurofascin and Ng-CAM, Drosophila neuroglian and zebrafish L1.1 and L1.2 (approximately 40% amino acid identity). Besides the high overall homology and conserved modular structure among previously recognized members of the L1 family (mouse/human L1/rat NILE; chicken Ng-CAM; chicken/mouse Nr-CAM; Drosophila neuroglian; zebrafish L1.1 and L1.2; chicken/mouse neurofascin/rat ankyrin-binding glycoprotein), criteria characteristic of L1 were identified with regard to the number of amino acids between positions of conserved amino acid residues defining distances within and between two adjacent Ig-like domains and FN-like repeats. These show a collinearity in the six Ig-like domains and four adjacent FN-like repeats that is remarkably conserved between L1 and molecules containing these modules (designated the L1 family cassette), including the GPI-linked forms of the F3 subgroup (mouse F3/chicken F11/human CNTN1; rat BIG-1/mouse PANG; rat TAG-1/mouse TAX-1/chicken axonin-1). The colorectal cancer molecule (DCC), previously introduced as an N-CAM-like molecule, conforms to the L1 family cassette. Other structural features of CHL 1 shared between members of the L1 family are a high degree of N-glycosidically linked carbohydrates (approximately 20% of its molecular mass), which include the HNK-1 carbohydrate structure, and a pattern of protein fragments comprising a major 185 kDa band and smaller fragments of 165 and 125 kDa. As for the other L1 family members, predominant expression of CHL1 is observed in the nervous system and at later developmental stages. In the central nervous system CHL1 is expressed by neurons, but, in contrast to L1, also by glial cells. Our findings suggest a common ancestral L1-like molecule which evolved via gene duplication to generate a diversity of structurally and functionally distinct yet similar molecules.